To apply an external language pack, send (or receive) the file via Telegram (attached as ‘file’), download it, then tap on it to see the menu (on Android: tap on … in the upper right corner of the doc bubble) then select ‘Apply localization file’. You will see a language selection menu with the language from the file as one of the options. Yes, this is damn tricky, we’ll simplify the process in the next versions.